1. Field of the Invention
This invention relates to a rotatable transformer field excitation system for an armature converter-fed, variable speed, self-controlled brushless synchronous motor having a wound field winding and a phase wound rotatable transformer-rotating rectifier wherein the primary winding of the rotatable transformer is energized by a voltage and current signal derived from the constant voltage of the alternating current source and from the variable current into the armature converter.
More particularly, the rotatable transformer excitation system utilizes a phase wound rotatable transfromer having a number of phases equal to the phases of the alternating current source, an electrical characteristic having its magnetizing reactance greater than the alternating current load resistance divided by the slip of the rotatable transformer and a current in the secondary winding thereof which is substantially independent of variations of transformer slip over a variable speed range.
2. Disclosure of the Prior Art
The use of a rotatable transformer-rotating rectifier excitation as a brushless method of excitation for an armature fed, variable speed, self-controlled synchronous machine is known in the art. Rotating transformer-rotating rectifier excitation of synchronous motors is described in an article entitled "Rotating-Rectifier Excitation for Synchronous Motors, Synchronous Condensers and Converter-Fed Synchronous Motors" by Gunther Kracke which appeared in the Siemens Review 37 (1970) No. 10, pages 530 to 534, inclusive.
The article in the Siemens Review likewise discloses the use of coaxial rotatable transformer with a rotating secondary winding-rotating rectifier for energizing the field winding. In such an arrangement, the coaxial primary windings of the rotating transformer are stationary, and the coaxial secondary windings rotate with the rotor. The coaxial windings are located in annular slots which are concentric with the axis of the rotor. Special design of the magnetic circuit with longitudinal and transverse stacking of the laminations is used as a means to insure the flux length with both windings encountering the same permeance in every position of the rotary part with respect to the stationary part to avoid short circuiting of the parts. The frequency is the same in both windings because the windings are coaxial. Thus, the same power can be transferred from the primary winding to the secondary winding at all times independently of the speed.
The Siemens article recognizes that, in a variable speed, converter-fed, synchronous motor which is required to operate at zero speed, a conventional exciter consisting of stationary d.c. field poles and a rotating phase wound armature connected to a rotating rectifier cannot be used to excite the field winding because the exciter output is zero at zero speed. Two solutions are suggested to overcome this problem. One solution was to utilize a phase wound rotatable transformer with the stator connected to the alternating current voltage source and the wound rotor connected to the rotating rectifier operated against the rotating field. If the synchronous machine is at zero speed, the rotatable transformer-rotating rectifier provides d.c. field current for the synchronous motor. However, as the motor speed increases, the rotor voltage and field excitation current both increase. In applications such as pump drives when the drive motor operates over a limited speed range of typically 70% to 100% of full speed, this variation in field excitation current may be acceptable. However, in applications such as traction drives, hoist drives, or actuator drives where the motor operates from 0% to 100% of full speed, the change in field excitation with speed caused by the voltage-fed phase wound rotatable transformer is unacceptable.
The second solution suggested in the Siemens article was to use an excitation transformer with a coaxial rotating secondary winding. Use of a coaxial rotatable transformer-rotating rectifier excitation source results in a voltage in the secondary winding of the rotatable transformer which is independent of speed. However, the coaxial rotatable transformer suffers the disadvantage of being expensive and difficult to manufacture.
In U.S. Pat. No. 3,866,099, Bourbeau suggested that the field winding could be fed by a rotatable transformer arrangement in a high frequency-to-low frequency converter system. In such a system having a large difference between the frequency applied to the rotatable transformer and the motor, variations in motor speed do not significantly affect the output of a phase wound rotatable transformer and, of course, have no effect on the output of a coaxial rotatable transformer.
In a variable speed, self-controlled, brushless synchronous motor, or application using the same, where the frequency of the power source is generally comparable to the maximum frequency of the synchronous motor, use of a voltage source to directly energize a phase wound rotatable transformer results in current in the rotatable transformer secondary winding which is dependent on and varies with motor speed.